1. Field of the Invention
The invention relates generally to the field of photography, and particularly to camera apparatus for preventing double exposure of an exposed film load.
2. Description of the Prior Art
In conventional 35 mm film manufacturers' cassettes, such as manufactured by Eastman Kodak Co. and Fuji Photo Film Co. Ltd., the filmstrip is wound on a flanged spool which is rotatably supported within a cylindrical shell. A leading of forward-most portion of the filmstrip approximately 21/3 inches long, commonly referred to as a "film leader", protrudes from a light-trapped slit or mouth of the cassette shell. One end of the spool has a short axial extension which projects from the shell, enabling the spool to be turned by hand. If the spool is manually rotated in an unwinding direction, the film roll inside the shell will tend to expand radially since the inner end of the filmstrip is attached to the spool, and the fogged leader portion protruding from the slit will remain stationary. The film roll can expand radially until a non-slipping relation is established between its outermost convolution and the inner curved wall of the shell. Once this non-slipping relation exists, there is a binding effect between the film roll and the shell which prevents further rotation of the spool in the unwinding direction. Thus, rotation of the spool in the unwinding direction cannot serve to advance the filmstrip out of the shell, and it is necessary in the typical 35 mm camera to engage the protruding leader portion to draw the filmstrip out of the shell.
A 35 mm film cassette has been proposed which, unlike conventional film cassettes, can be operated to automatically advance a film leader out of the cassette shell by rotating the film spool in the unwinding direction. The film leader originally is located entirely within the cassette shell. Specifically, in U.S. Pat. No. 4,423,943, granted Jan. 3, 1984, there is disclosed a film cassette wherein the outermost convolution of the film roll wound on the film spool is radially constrained by respective circumferential lips of two axially spaced flanges of the spool to prevent the outermost convolution from contacting an inner wall of the cassette shell. The trailing end of the filmstrip is secured to the film spool, and the forward or leading end of the filmstrip is reduced in width to allow it to protrude from between the circumferential lips and rest against the shell wall at a location inwardly of a film passageway to the outside of the cassette shell. During initial unwinding rotation of the film spool, the leading end of the filmstrip is required to "seek" the entrance to the film passageway. Then, it is advanced into and through the film passageway to exit the cassette shell. If the filmstrip is rewound midroll into the cassette shell, i.e., before all of the available imaging areas are exposed, because the photographer wishes to change the film type, the film leader will be returned to its original location inwardly of the film passageway. Thus, in order to re-use the filmstrip to expose its remaining imaging areas, the leading end of the film leader will be required again to seek the entrance to the film passageway to exit the cassette shell.